Ithiriel // Thoughts From a Sysadmin And Occasional Coder

This is a reasonably simple process. Most of the process can be found in this Enigmail forum discussion.

1. Move the GnuPG keys to a USB drive. (For the purpose of this discussion, I will assume that the USB drive is X: and the directory on the drive is .gnupg.)
2. On the computer (not on the USB drive), change gpg.conf to include these directives:
   

   no-default-keyring
   keyring X:\.gnupg\pubring.gpg
   primary-keyring X:\.gnupg\pubring.gpg
   secret-keyring X:\.gnupg\secring.gpg
   trustdb-name X:\.gnupg\trustdb.gpg

   Under Mac OS X, assuming a volume name of USB drive, you would add:

   no-default-keyring
   keyring /Volumes/USB drive/.gnupg/pubring.gpg
   primary-keyring /Volumes/USB drive/.gnupg/pubring.gpg
   secret-keyring /Volumes/USB drive/.gnupg/secring.gpg
   trustdb-name /Volumes/USB drive/.gnupg/trustdb.gpg

   For Linux, it should be the same as for OS X but /Volumes/USB drive would be replaced by the mount point used for the drive.

3. And that's it.

If you want to use an encrypted partition or filestore, e.g. through TrueCrypt, the above instructions are still valid. However, you would point it to wherever you have TrueCrypt mount the encrypted partition or filestore.